The present invention relates to an exhaust aftertreatment apparatus for an internal combustion engine, and more particularly to a diagnosis system of the exhaust aftertreatment apparatus.
In case of a lean-burn engine capable of combusting air-rich mixture, an adsorption and oxidization type NOx catalyst (NOx trap catalyst) is mostly provided in an exhaust passage of the engine to purify (deoxidize) nitrogen oxide (NOx) in exhaust gas. Such a NOx trap catalyst adsorbs NOx in exhaust gas when an air-fuel ratio in exhaust gas is high (lean), and desorbs and deoxidizes the trapped NOx when the air-fuel ratio is low (rich). Due to the limitation of the adsorbing capacity of NOx trap catalyst, it is necessary to desorb and deoxidize the trapped NOx at a proper timing before the adsorbing capacity is saturated. Therefore, a known exhaust aftertreatment apparatus is arranged to execute a rich spike control of temporally lowering the exhaust air/fuel ratio as disclosed in Japanese Published Patent Application No. 7-279718.
On the other hand, there is a need of accurately detecting the abnormality of a NOx trap catalyst to maintain the high exhaust purifying efficiency. In order to satisfy such a need, Japanese Published Patent Applications No. 2002-38929 and 2001-73747 discloses a diagnosis method of measuring an exhaust air/fuel ratio during the rich spike control and diagnosing a deterioration of the NOx trap catalyst on the basis of a time period during when the measured exhaust air/fuel ratio is maintained at ratios near the stoichiometric air/fuel ratio.
Further, Japanese Published Patent Application No. 10-121943 discloses a diagnosis method of measuring an inversion cycle of an output of an O2 sensor during a feedback control of an air/fuel ratio of mixture supplied to the engine according to the output of the O2 sensor disposed downstream of the catalyst, and of diagnosing the deterioration of the catalyst from the measured inversion cycle.